Het voedsel van de Kerkuil Tyto alba in Drenthe

Remains of prey items found in pellets of Barn Owls from the province of Drenthe were analysed. The validity of this material is restricted, because many sets of pellets were not collected on a regular (i.e. monthly) basis. Moreover, pellets from one pair found at different sites (bam, shed, nestbox) were normally lumped, despite striking discrepancies in the proportion of Common Voles relative to site (Table 1). The province of Drenthe mostly consists of farmland (76%), followed by woodland (11%) and heaths (6%). Farming practices are intensive, both on arable land (45% of total area of farmland) and on grassland (46%). Drenthe is -compared with other provinces in The Netherlands- thinly populated, with on average 164 inhabitants/100 ha. Bam Owls in Drenthe mainly prey on Common Voles (40% of all prey items) and Common Shrews (36%), which is typical for Dutch Barn Owls (Table 2). The high proportion of Common Voles in Drenthe, as compared to The Netherlands, is caused by the preponderance of material from 1988 and 1989 (Appendix 1), years with an outbreak of Common Voles. Seasonal changes in diet are pronounced, with Common Shrews featuring prominently in the diet in December through May and Common Voles from June through November (Table 3). Annual variations in diet are mainly caused by the cyclic pattern of vole abundance. High numbers of Common Voles were present in 1988-90, and this is reflected in the diet. Low vole numbers are accompanied by a high proportion of Common Shrews in the diet, as in 1986 (Fig. 1, Table 4). Common Voles are an important food source, as found in the increase in Drenthe’s Barn Owl population and the high number of fledglings per nest during vole years (Table 5).